1. Technical Field of the Invention
The present invention relates to cellular telephone systems and, in particular, to a method for allocating frequencies to individual cells of a cellular telephone system.
2. Description of Related Art
Cellular telephone systems divide a large service area into a number of smaller discrete geographical areas called "cells" each typically ranging in size from about one-half to about twenty kilometers in diameter. Each cell is at least contiguous and/or overlapping with multiple adjacent cells to provide substantially continuous coverage throughout the service area. A base station including a plurality of transceivers capable of operating independently on different assigned radio frequencies is provided for each of the cells. Via the transceivers, the base stations engage in simultaneous communications with plural mobile stations operating within the area of the associated cell. The base stations further communicate via data links and voice trunks with a central control station, commonly referred to as a mobile switching center, which functions to selectively connect telephone calls to the mobile stations through the base stations and, in general, control operation of the system.
Each cell is allocated use of a predetermined set of frequencies from the cellular frequency band for use in providing its control and voice/data (traffic) channels. The allocation is typically made in accordance with a certain frequency plan. The frequencies used for the control and traffic channels assigned to a given cell are preferably spaced apart from each other across the frequency spectrum of the cellular frequency band. This serves to minimize the instances and adverse affects of adjacent channel interference.
Because only a limited number of frequencies are available in the cellular frequency band, the same frequencies that are allocated to one cell are also allocated to (i.e., reused by) other cells in distant parts of the service area. Typically, adjacent cells are not allocated to use the same frequency by the frequency plan. Furthermore, the power levels of the signal transmissions on any given frequency are limited in strength so as to limit propagation beyond the cell area. The foregoing precautions serve to reduce instances of co-channel interference caused by reuse of that same frequency in a distant cell. It is further noted that careful power level and distance allocation also assists in reducing instances of adjacent channel interference.
In spite of the precautions taken by service providers in the frequency plan allocation for a frequency reuse cellular telephone system and in the regulation of system operation, it is known that instances of co-channel interference do occur. This interference may be affected by a number of factors including: terrain irregularities; radio propagation changes; fading; multipath propagation; sectorization; reflection; existence of human and natural obstructions; the number of available transceivers per cell; and variations in demand. This interference often adversely affects system operation by, for example, degrading voice quality on the traffic channels or interfering with the transmission and reception of control signals on the control channels. Service providers accordingly invest a substantial amount of effort in frequency planning for optimal system operation.
The conventional frequency planning process utilizes software tools for predicting, for each frequency, the interference resulting from concurrent use of that frequency by other cells. This process is implemented by the service provider not only when the system is initially set up, but also at each instance thereafter when new cells or equipment are added. The interference predictions made by the frequency planning software tools rely on certain wave propagation models and other theoretical considerations. It is often difficult to construct a reasonably accurate wave propagation model and take into account other theoretical considerations for complicated environments (such as urban areas) which are subject to the effects of terrain irregularities, multipath propagation, sectorization, reflection and the existence of obstructions. Accordingly, software tool directed frequency planning alone (i.e., without the support of the cellular telephone system) is of limited practical benefit in many cases.
A substantial amount of effort has been directed toward involving the cellular telephone system itself in the frequency planning process. These efforts have primarily focused on collecting more and more interference related information from the system for service provider analysis and use in determining an optimal speech quality frequency plan. For example, service providers now routinely program their cellular telephone systems to have base stations make and report uplink signal strength measurements on an idle frequency by frequency basis. The collected uplink idle frequency signal strength measurements are indicative of the uplink interference on each frequency, and the measurements accordingly provide valuable information useful in identifying shortcomings of an existing frequency plan and proposing needed reallocations.
The most recent evolution in frequency planning is a concept for supporting a more or less automatic frequency planning scheme implemented by the cellular communications system itself with a minimum of service provider management and oversight. This concept is generally referred to as adaptive frequency allocation (AFA). The basic operation of the adaptive frequency allocation concept is to measure the interference in all cells and on all frequencies, and then utilize the measured data to iteratively reallocate frequencies within the cellular communications system to provide for optimal speech quality. Reallocation decisions may take into account either the interference in a single cell (providing a "localized" approach) or the interference in a number of cells (providing a "centralized" approach).
Situations often arise where the signal strength measurements indicative of uplink interference are misleading. For example, due to the directional nature of the antennas utilized in sectorized cellular systems, the uplink on a given frequency might indicate little interference concerns while the downlink is severely interfered due to the directional broadcasts from a neighboring cell. The making of frequency allocation decisions, especially when using an adaptive frequency allocation scheme, based solely on uplink signal strength measurements is accordingly not recommended.
It is preferred that both uplink and downlink interference be measured and considered in the context of any frequency allocation determination. A number of limitations arise when attempting to collect downlink interference information. Ideally, and conveniently, the downlink interference measurements should be made by mobile stations operating within the cellular communications system. Current system specifications (such as that defined for the Global System for Mobile (GSM) communications) support mobile station downlink signal strength measurements only on those broadcast control channel (BCCH) frequencies of neighboring cells specified by a mobile station received neighbor (BA) list. These measurements provide some information indicative of downlink interference on those frequencies. No support, however, is provided for the mobile station making downlink interference measurements on the BCCH frequency allocated to the currently serving cell as the mobile station cannot separate emissions from its own cell from the emissions of other cells. There is a need for a mechanism to collect downlink interference information for general use in frequency planning, and in particular for use in connection with adaptive frequency allocation schemes.